Sub name matches filename with extension .macro.
Added core event for handling special gcode comments used by expressions and flow control: DEBUG, PRINT and ABORT.
Added overrideable default $-setting values for second PWM spindle to grbl/config.c.
Added support for LinuxCNC style (ABORT,<msg>) comment, requires expressions enabled. Terminates gcode program, outputs message and returns error 253.
Added PRM[<setting>] and PRM[<setting>,<bit>] functions to expressions, returns $-setting value or value of bit in integer type setting.
"hardened" flow control code, fixed bug in repeat...continue handling.
Changed signature of grbl.on_gcode_comment event, now returns status code.
Added clear of return value on CALL statement and added optional return value expression support to ENDSUB.
Improved handling of multiple simultaneous spindles. Still work in progress.
For developers: moved user M-code entry point hooks from the HAL structure to the core handlers and changed signature of the check() function to better support valueless words (letters) when parameter support is enabled. Removed deprecated parameter from the validate() call signature.
Added $384 setting for controlling G92 offset persistence.
Improved $help command output and handling.
Moved the optional tool table in non-volatile storage.
Added gcode parameter support and optional expression support.
